1. Field of the Invention
The present invention relates to the cooling of turbine components in a gas turbine engine and, more particularly, to a variable radial flow inducer for modulating the cooling air flow to such turbine components.
2. Description of Related Art
Gas turbine engines typically include cooling systems which provide cooling air to turbine rotor components, such as turbine blades, in order to limit the temperatures experienced by such components. Prior art cooling systems usually acquire the air used to cool turbine components from the engine's compressor, after which it is diverted and subsequently directed to the turbine section of the engine through an axial passageway. A device commonly known as an inducer is generally located at the exit end of such an axial passageway in order to accelerate the airflow before it impinges on the turbine components to be cooled. Such inducers, frequently in the form of a circumferentially disposed array of vanes, are used to control the tangential speed of the airflow so that it is substantially equal to that of the turbine rotor. An exemplary inducer utilized for such purpose is disclosed in U.S. Pat. No. 4,882,902 to James R. Reigel et al., entitled "Turbine Cooling Air Transferring Apparatus." Another inducer performing a similar function to the vane-type inducer is disclosed in U.S. Pat. No. 5,245,821 to Theodore T Thomas Jr et al entitled "Stator to Rotor Flow Inducer," where a plurality of cylindrical airflow passages are disposed circumferentially about the engine centerline.
An important factor in the design of cooling systems is its relationship to the efficiency of the gas turbine engine. In current prior art systems, the amount of cooling flow is generally fixed at a level required to achieve requisite cooling at the maximum turbine inlet temperature point for the engine. Since an engine is usually run at conditions which are less than maximum turbine inlet temperature, this causes the engine to normally operate with excess cooling flow and decreased efficiency. This excess cooling also has the effect of increasing overall engine specific fuel consumption. Accordingly, an apparatus capable of modulating the flow of cooling air through an inducer to the turbine components in response to changes in the turbine inlet gas temperature would increase the efficiency of the gas turbine engine and be most desirable.
As seen in U.S. Pat. No. 4,807,433, some turbine cooling systems modulate the amount of cooling air provided to the turbine in accordance with the engine cycle. These systems, however, are inefficient because they modulate the airflow at the point where diverted from the compressor, resulting in flow losses prior to the air reaching the turbine. While it is also known for cooling systems in the prior art to employ an axial flow inducer at the exit of the cooling flow path, modulation of the cooling airflow at this axial location has been avoided due to design complexity. Consequently, there exists an unfulfilled need for an apparatus which modulates the flow of cooling air to the turbine at the exit of the cooling flow system